Electrostatic discharge pulses are a well known phenomenon and can be generated by any of a number of causes. These pulses, which can be several thousand volts, are known to destroy integrated circuits and especially integrated circuits incorporating field effect transistors (FETs). Accordingly, integrated circuits frequently provide some sort of protective circuit for preventing high voltages applied to an input pad from reaching an input to a device to be protected, such as an input buffer. An example of a prior art circuit for protecting against electrostatic discharge is shown in FIG. 1, where diodes D1 and D2 prevent voltages from being applied to an input buffer which are significantly above power supply voltage V.sub.DD or significantly below ground potential. In the example of FIG. 1, if a positive electrostatic pulse is applied to input pad 10, this high voltage will be shunted to the supply voltage through diode D1 so that the maximum voltage appearing at node 1 will be approximately the supply voltage plus the diode drop of diode D1. Resistor R1 connected between node 1 and input pad 10 serves to limit current through diode D1. Any negative voltages applied to input pad 10 will be shunted to ground through diode D2.
In some cases a manufacturer or a user of an integrated circuit may want to apply a voltage to an input pad which is significantly above the power supply voltage in order to enable various test functions of the integrated circuit which are otherwise unaccessible by applying normal input levels to the input pads. In this case, the electrostatic discharge protection circuit of FIG. 1 could not be utilized in the integrated circuit since any voltage significantly above power supply voltage V.sub.DD would forward bias diode D1 and prevent node 1 from rising significantly above power supply voltage V.sub.DD. A different structure is needed which will allow input levels to be driven high enough above V.sub.DD to enable other test modes, but will still provide sufficient protection against positive electrostatic discharge pulses applied to the input.